1. Technical Field of the Invention
The present invention relates to a current collector for use in a secondary battery, particularly in a lithium secondary battery or lithium-ion battery, and to a manufacturing process thereof.
2. Prior Art
A secondary battery essentially comprises a positive electrode, a negative electrode, a separator for insulating the positive electrode and the negative electrode from each other, and an electrolyte for making it possible to move ions between the positive electrode and the negative electrode. The positive electrode and the negative electrode are formed by coating the surface of a current collector of metal foil with an optional active material. For example, in the lithium ion battery, a current collector of aluminum foil coated with an active material containing lithium cobaltate, etc. is used as a positive electrode, and a current collector of copper foil coated with an active material containing non-graphitizable carbon, etc. is used as a negative electrode.
Generally, there has been a problem that when coating a surface of a metal foil such as aluminum foil or copper foil with active material the metal foil and the active material are difficult to integrate or attach to each other, and the active material is relatively easy to drop out. In the preparation of a secondary battery, if a part of the active material drops out at the time of winding the positive electrode and the negative electrode, there arises a problem of not being able to obtain a secondary battery of a desired capacity. If a part of the active material drops out after the preparation of the secondary battery, there arises another problem that the charge and discharge capacity of the secondary battery is gradually reduced.
To cope with this problem, it has been conventional to use a binder which in the active material has a good affinity with the metal foil. Further, it has been also conventional to use a metal foil a surface of which has a good affinity with an optional binder. For example, the Japanese Laid-Open Patent Publication (unexamined) Hei 7-201332 discloses a technique in which an azole film such as benzotriazole is formed on a surface of a copper foil, thereby improving the integration between a binder of the active material and the copper foil, and preventing the active material from dropping out.
On the other hand, being different from the mentioned method, another technique is also known, in which the active material is prevented from dropping out by forming penetrating holes through the metal foil and integrating the active material for coating front and back sides of the metal foil through these penetrating holes. It is certain that, in this method, as the active material, etc. on both sides are integrated through the penetrating holes, the active material, etc. are effectively prevented from dropping out. But the adherence between each periphery or each inner wall of the penetrating holes and the active material, etc. or adherence between the metal foil and the active material, etc. still remains insufficient, and therefore if a large external force is applied, there is a possibility that a part of the active material will drop out.